There is a need for improved high-power pulse generation for a number of applications, including LIDAR (light detection and ranging) and in particular LADAR (laser detection and ranging) for detecting objects (particularly flying objects) at great distances, as well as for medical treatments and materials processing. There is also a need for high average power continuous-wave (CW) sources capable of narrow-line operation for applications such as directed energy.
Optical amplifying fibers can have very high gain. When pumping such fibers to very high powers, the high power per area in the core causes substantial non-linear effects such as stimulated Brillouin scattering (SBS) and four-wave mixing.
In recent years, fiber lasers have deployed in applications requiring increasing average power, peak power and pulse energy, often while also requiring near-diffraction-limited beam quality. Large-mode-area fibers (LMA) have been the key enabling technology. These fibers are usually intrinsically multimode but operate on the lowest-order mode based on a combination of fundamental-mode excitation (as described in U.S. Pat. No. 5,818,630 titled “Single-mode amplifiers and compressors based on multi-mode fibers” to Fermann et al., which is incorporated herein by reference) and preferential bend-loss for higher-order modes (HOMs) (as described in U.S. Pat. No. 6,496,301 titled “Helical fiber amplifier” to Koplow et al., which is incorporated herein by reference). Alternatively, photonic crystal fibers or otherwise micro-structured fibers enable genuine single-mode operation of LMA designs (Mortensen et al, “Low-loss criterion and effective area considerations for photonic crystal fibres,” J. Opt. A: Pure Appl. Opt. Vol. 5 163-167 (2003); Nielsen et al., “Low-loss photonic crystal fibers for transmission systems and their dispersion properties,” OPTICS EXPRESS Vol. 12, No. 7 pp. 1372-1376 (2004); DiTeodoro et al., “MW peak-power, mJ pulse energy, multi-kHz repetition rate pulses from Yb-doped fiber amplifiers,” Proceedings of SPIE—Vol. 6102, Fiber Lasers III: Technology, Systems, and Applications, Andrew J. W. Brown, Johan Nilsson, Donald J. Harter, Andreas Tünnermann, Editors, 61020K (published online Feb. 23, 2006) (Photonics West 2006)). The price of this single-mode operation is fundamental mode bend-loss, so conventional extreme micro-structured designs have evolved into rods (Di Teodoro et al., “Very large-core, single-mode Yb-doped photonic crystal rod for multi-MW peak power generation,” Proceedings of SPIE—Vol. 6102, Fiber Lasers III: Technology, Systems, and Applications, Andrew J. W. Brown, Johan Nilsson, Donald J. Harter, Andreas Tünnermann, Editors, 61020D (published online Feb. 23, 2006)).
An optical glass “preform” is the source material from which an optical fiber is drawn, typically by heating the preform and pulling glass from the bottom to form the optical fiber. One method of preform fabrication is the “rod-in-tube” method such as is described in U.S. Pat. Nos. 4,668,263 and 4,264,347, which are incorporated herein by reference. A rod of glass that will form the core of the fiber is inserted into a thick-walled tube (or a plurality of concentric tubes) that will become the cladding(s), and these are fused together at high temperature to form the preform. When the fiber is drawn from the heated preform, the relative dimensions of the core and cladding in the drawn fiber are identical to those of the original preform, which takes its shape from those of the rod and tube(s). Such methods do not allow one side of the cladding to have a different index than another side of the cladding, nor do they allow one side of the core to have a different refractive index or active-species doping than another side of the core.
U.S. Pat. No. 6,711,918 titled “Method of bundling rods so as to form an optical fiber preform” issued to Kliner and Koplow on Mar. 30, 2004, and is incorporated herein by reference. This patent describes one way for fabricating fiber-optic glass preforms having complex refractive-index configurations and/or dopant distributions in a radial direction with a high degree of accuracy and precision. Their method teaches bundling together a plurality of glass rods of specific physical, chemical, or optical properties and wherein the rod bundle is fused in a manner that maintains the cross-sectional composition and refractive-index profiles established by the positions of the rods. This patent does not describe sloping the refractive index from one side of the cladding to another side of the cladding, nor having a different refractive index or active-species doping profiles on one side of the core than another side of the core.
U.S. Pat. No. 7,209,619 titled “Photonic bandgap fibers” to Dong et al., U.S. Pat. No. 5,818,630 titled “Single-mode amplifiers and compressors based on multi-mode fibers” to Fermann et al., and U.S. Pat. No. 6,496,301 titled “Helical fiber amplifier” to Koplow et al., are each incorporated herein by reference. These describe apparatus and methods that may be combined and/or modified using the teachings of the present invention.
U.S. patent application Ser. No. 11/426,302 (now U.S. Pat. No. 7,526,167) titled “APPARATUS AND METHOD FOR A HIGH-GAIN DOUBLE-CLAD AMPLIFIER” was filed Jun. 23, 2006 by John D. Minelly, and is incorporated herein by reference. U.S. patent application Ser. No. 11/567,740 (now U.S. Pat. No. 7,570,856) titled “APPARATUS AND METHOD FOR AN ERBIUM-DOPED FIBER FOR HIGH PEAK-POWER APPLICATIONS” was filed Dec. 7, 2006 by John D. Minelly et al., and is incorporated herein by reference. U.S. patent application Ser. No. 11/556,658 (now U.S. Pat. No. 7,400,807) titled “APPARATUS AND METHOD FOR A WAVEGUIDE WITH AN INDEX PROFILE MANIFESTING A CENTRAL DIP FOR BETTER ENERGY EXTRACTION” was filed Nov. 3, 2006 by John D. Minelly et al., and is incorporated herein by reference. U.S. patent application Ser. No. 11/420,729, (now U.S. Pat. No. 7,391,561) titled “Fiber- or rod-based optical source featuring a large-core, rare-earth-doped photonic-crystal device for generation of high-power pulsed radiation and method” issued to Fabio Di Teodoro et al. Jun. 24, 2008, and is incorporated herein by reference. U.S. Pat. No. 7,199,924 titled “Apparatus and method for spectral-beam combining of high-power fiber lasers” issued to Andrew Brown et al. Apr. 3, 2007, and is incorporated herein by reference. Each of these patents and applications are assigned to the assignee of the present invention. Each describes various features of high-power optical-fiber amplifiers that, in some embodiments, are combined with some embodiments of the present invention. Some of the various embodiments of the above-described inventions describe large-mode-area (LMA) cores in optical fibers or rods that support single-mode signals, but wherein the fibers or rods must be kept very straight to avoid bend losses.
Improved apparatus and methods are needed to generate high-power optical pulses from curved, coiled, and/or bent waveguides, such as optical fibers.